FIG. 1 shows a diagram of a typical radio receiver 100. The receiver 100 includes a switched-gain low noise amplifier (LNA), a phase-locked loop (PLL) that includes a voltage-controlled oscillator (VCO) and drives a radio frequency down-converting mixer 102, a bandpass filter (BPF), a variable gain amplifier (VGA), an I/Q demodulator, a low pass channel-select filter, and analog to digital (A/D) converter. The radio receiver uses an automatic gain control (AGC) loop to adjust the switched-gain LNA and VGA in a way that compensates for variations in the power of the received signal.
The performance of the radio receiver depends on the operating bias of key circuits such as the LNA, RF down-converting mixer, and VCO. This is simply because the available output power from these circuits depends on their supply voltage (Vsupply) and bias current (lbias), i.e.,Pout≦VsupplyIbias while the compression point, P-ldB, and the third order intercept point, IP3, track the available output power. As a result, to improve linearity generally requires increasing circuit's operating bias.
Radio receivers are designed to meet minimum performance requirements. These requirements ensure the radio will operate in most situations—even weak-signal environments subject to strong interfering signals that can lead to cross modulation, reciprocal mixing, and intermodulation problems. But this means key circuits in the radio receiver oftentimes require a high operating bias, even though the likelihood of these difficult situations is low. As a result, the radio receiver performs much better and operates at a higher bias current than needed.
Most RF circuits use feedback to set their gain and performance. In these circuits, the operating bias defines the available output power and linearity. The operating current does not, however, affect the gain of the circuit. Therefore, it is practical to adjust the operating current for these circuits without disturbing the AGC loop and any power control algorithms. Since power consumption is a critical issue in portable radios, it would be advantageous to a system that adjusts the operating current of the radio based on the performance needed.